1. Field of the Invention
The present invention generally relates to electronic circuits and more specifically to so-called switched-mode power converters, also called switched-mode power supplies.
2. Discussion of the Related Art
A switched-mode converter uses inductive elements, associated with a power switch and with a free wheel diode, to perform a power conversion, generally from a generally D.C. input voltage, or from an A.C. voltage when the power factor is desired to be corrected. This input voltage is most often obtained by rectification of an A.C. voltage, typically the mains voltage of the electric distribution network. The converter provides a D.C. voltage regulated with respect to a set-point value. Voltage step-down (buck), step-up (boost) and mixed (buck-boost) converters are known.
Converters generally comprise a switching-aid circuit and a control circuit of the power switch. The control circuit control the duty cycle of the chopping of the input voltage so that the output voltage corresponds to the desired reference value. The switching-aid circuit especially aims at decreasing power losses.
FIG. 1 shows a simplified diagram of an known example of a boost converter associated with its switching-aid circuit. Such a converter with a switching-aid circuit is described in U.S. Pat. No. 6,987,379.
This boost converter comprises a switch K controlled by a circuit 1′ (CTRL), for example, a pulse-width modulation control circuit (PWM) based on a desired reference voltage REF and on a measured value FB of the D.C. output voltage Vout provided to a load 2 (Q). A power storage inductance L0 is connected, by a first terminal 21, to a first terminal of application of a rectified A.C. input voltage. Terminal 21 generally corresponds to the positive rectified output voltage of a rectifying bridge 3 having its A.C. input terminals connected to terminals 31 and 32 of application of an A.C. supply voltage Vac (for example, the mains voltage). A second terminal 22 of inductance L0 is connected to a terminal 23 of application of a reference voltage, by a di/dt control inductance L in series with switch K. The reference voltage is common to the D.C. input and to the output of the converter. This voltage generally corresponds to the ground, connected to the reference rectified output terminal of bridge 3.
Inductance L0 belongs to a magnetic circuit 4 and forms its main winding. Circuit 4 comprises two secondary windings L1 and L2 having numbers of spirals, respectively N1 and N2 smaller than number N0 of spirals of inductance L0. A first winding L1 of magnetic circuit 4 is connected between inductance L0 and a free wheel diode DL having its cathode connected to a positive output voltage 24 of the converter. Inductance L1 may also be placed between the cathode of diode DL and terminal 24. A second winding L2 connects node 22 to reference terminal 23 by being in series with a diode D2, the anode of diode D2 being on the side of ground 23. The respective positions of diode D2 and of inductance L2 may also be inverted. Finally, a diode D1 connects node 25 of interconnection between di/dt control inductance L and switch K to terminal 24, the anode of diode D1 being on the side of node 25. Terminal 24 is connected to reference terminal 23 by a storage capacitor C across which is present regulated output voltage Vout. Capacitor C may belong to the load to be supplied by the power converter. In the example of FIG. 1, assuming that the phase point of winding L0 is connected to terminal 21, the phase point of winding L1 is on the side of terminal 22 and the phase point of winding L2 on the side of ground 23. If however, the phase point of winding L0 is on the side of terminal 22, the phase point of winding L1 needs to be on the side of terminal 24 and the phase point of winding L2 on the side of terminal 22.
When switch K is turned on, the power is stored in inductance L0 and load Q is supplied by the power stored in capacitor C. When switch K is off, inductance L0 gives back stored power to capacitor C via free wheel diode DL.
The function of inductance L is to limit, on the one hand, the dissipated losses resulting from the voltage decrease and with the current increase in switch K at its turning-on and on the other hand, the losses resulting from free wheel diode DL due to a recovered charge phenomenon.
The function of magnetic circuit 4 is to recover the recovery current of diode DL to transfer it into main inductance L0, so that it is then provided to the load. For this purpose, winding L1 imposes, at the turning off of switch K, a negative voltage across inductance L to enable it to transfer the power that it contains to capacitor C. Diode D1 is then forward biased. Winding L2 has the function, at the turning-on of switch K, to impose a negative voltage across inductance L, to transfer the power that it contains into winding L2 of the magnetic circuit. This power is then recovered by winding L0 which gives it back to capacitor C at the next turning-off of the switch.
Switch K is generally formed of a MOS transistor and exhibits a stray capacitance CK at its terminals (between its drain and source). This stray capacitance generates losses at each turning-off and turning-on of the switch. At the turning-on, the power stored in the stray capacitance is dissipated in the switch (in its on-state series resistance). At the turning-off, the stray capacitance stores power, causing new losses in the switch. Voltage peaks generate noise and wear out the switch.
It would be desirable to further improve the operation of a power converter to decrease its switching losses and protect the cut-off switch.